This is the absolute last day you can pay your 2016 taxes

The last day to pay your 2016 federal income taxes is April 18, 2017, a date commonly known as Tax Day.

This payment deadline applies whether you file a tax extension or not. Here's what you need to know about the tax deadline, as well as what it will cost if you pay late.

Tax Day 2017

Tax Day in the United States generally falls on April 15, unless that date falls on a weekend or holiday. In 2017, thanks to a weekend and a holiday (Emancipation Day -- a D.C. holiday), Tax Day is pushed back to April 18.

So, the absolute last day you can pay your 2016 federal income taxes is April 18, 2017

But I filed a tax extension. Don't I have until October?

It's a popular misconception that filing a six-month tax extension delays your payment due date until mid-October. This is 100% false.

A tax extension simply delays the date by which your tax return paperwork must be submitted by six months. The point of this is to allow extra time for people who still need to gather income documentation, track down receipts, or otherwise just need additional time to fill out their tax returns.

It does not extend the payment deadline. Any balance owed to the IRS is still due on the April 18 Tax Day. In fact, if you're not finished with your return and plan to request an extension, you're expected to estimate the amount of tax you'll owe, and send this payment in with your extension paperwork. Failure to do so can result in interest and penalties.

Are there any exceptions to the April 18, 2017 deadline?

There are a few exceptions to the deadline to pay and file taxes, such as:

If you're in the Armed Forces and are serving in a combat zone.

If you're a citizen or resident alien working abroad.

If you were a victim of certain disaster situations. You can see recent examples, and the deadlines that were affected, here.

If any of these affect you, the relevant publications are available on www.irs.gov, or you can consult a tax professional to determine if the deadlines don't apply to you.

Penalties for paying (and filing) late

Technically speaking, I should say that April 18, 2017, is the absolute last day to pay your taxes without paying interest and penalties. It's not exactly a payment cut-off date -- quite the contrary; the IRS will gladly accept a payment after Tax Day has passed. However, depending on how late you are and how much you owe, the IRS can assess interest and penalties on your unpaid balance.

Interest compounds daily on any tax that remains unpaid as of the due date. The IRS interest rate can change quarterly, and as of this writing, the rate is 4%.

Penalties depend on whether you simply haven't paid your taxes, or if you haven't filed at all. And the latter is much worse. The failure to pay penalty is 0.5% per month, or part of a month, up to a maximum of 25% of the unpaid tax amount. The failure to file penalty, on the other hand, accumulates 10 times as fast -- 5% per month, or part of a month, up to the same 25% maximum. So, even if you can't pay, it's a smart idea to file your tax return anyway.

As an example, let's say that you owe the IRS $5,000, and that your balance remains unpaid for one year after the deadline. Assuming the interest rate stays the same, you'll owe roughly 4% of the balance for interest (slightly more, since it compounds daily). And, you'll owe a total failure to pay penalty of 6% of the balance, calculated as 0.5% per month for 12 months. This translates to roughly $500 tacked on to your tax bill for a year of non-payment.

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